1. Field of Invention
The present invention relates to a method and apparatus for controlling the concentration of an additive in the recirculating water of a cooling tower system.
2. Description of the Prior Art
In a cooling tower system, it is important to maintain a constant concentration of additives in the recirculating liquid, which additives prevent corrosion to the components of the cooling tower.
Ordinary ground water is primarily used as the cooling medium in many types of cooling towers Corrosive elements, such as calcium and magnesium, are thus present in the recirculating water of the cooling tower. As the water evaporates, the concentrations of the calcium and magnesium increase. The calcium and magnesium in high concentrations and over a period of time cause corrosion to the cooling tower system and in particular to the metallic parts of the system. For this reason, chemicals are added to the recirculating water to decrease the effect of the calcium and magnesium present in the water.
A typical cooling tower system is shown schematically in FIG. 1. In the conventional system, control of the additive concentration in the cooling tower is effected with a high-limit conductivity sensor. As its name implies, the conductivity sensor senses the conductivity of the recirculating water in the cooling tower. The conductivity of the water is directly proportional to the increased concentration of the corrosive elements in the water (i.e., the calcium and magnesium).
Most conventional cooling towers of this type include a blow-down valve, which valve removes recirculating water from the cooling tower system. Make-up water is added through another conduit, and the make-up water is controlled by a float sensor, for example, which detects the proper level of water in the tower.
In conventional cooling tower systems of the type shown in FIG. 1, the blow-down valve is actuated in response to a signal from the conductivity sensor which detects a high concentration of corrosive elements in the recirculating water. When the blow-down valve is actuated, a quantity of recirculating water is removed from the cooling tower system. When the water in the cooling tower falls below a predetermined level detected by the float sensor, a make-up water valve is actuated to replenish the supply of water in the cooling tower.
In the conventional cooling tower system, an additive pump (and possibly an associated valve) is provided, which pump when actuated introduces an anti-corrosion additive to the cooling tower. The additive mixes with the water recirculating in the cooling tower system. The additive pump is actuated in response to the same signal that actuates the blow-down valve. In other words, the additive pump operates to add the additive to the cooling tower only during the time that the blow-down valve is open. The pump shuts down at the same time that the blow-down valve closes; this occurs when the conductivity detected by the conductivity sensor falls below a predetermined value.
A particular concentration of additive must be present in the recirculating water of the cooling tower system in order to prevent corrosion to the system components. However, the method of adding anti-corrosion additives for the same period of time that the blow-down valve is open and in response to the signal from the conductivity sensor is an imprecise way of controlling the concentration of additive in the recirculating water. The blow-down valve, through which recirculating water is removed from the system, and the make-up water valve, through which replenishing water is added, are actuated independently of each other, the former by the conductivity sensor and switch, and the latter by the float sensor which detects the water level in the cooling tower. Thus, when the blow-down valve is off, the make-up water valve may still be on, with water continuing to flow into the tower. Accordingly, it is possible that the additive pump, which shuts down when the blow-down valve closes, stops pumping additive into the cooling tower prematurely, that is, while the make-up water is still being added to the cooling tower. Thus, the concentration of additive in te recirculating water of the cooling tower system may fall below a level that is sufficient to inhibit corrosion of the system components.
Furthermore, if water from the recirculating water evaporates, the make-up water solenoid valve will be energized even though blow-down has not occurred. Since in the conventional system the additive pump is tied directly to the same circuit that controls blow-down, no anti-corrosion additive is introduced into the system to mix with the make-up water that is added to replenish the recirculating water that has evaporated. Therefore, the concentration of the corrosion additive in the recirculating water decreases.